Abstract: The discovery of tumor stem cells in human brain tumors has greatly changed the biological and clinical views of treatment-refractory brain cancer. Targeted therapies causing tumor stem cells to differentiate, undergo apoptosis, or die, therefore represent a novel therapeutic strategy to treat recurrent tumors. The goals of this proposal are to identify genes that confer the tumor stem cell quiescence, and to develop new brain tumor therapies based on the blockade of cellular quiescence in order to potenciate the treatment efficacy of radiation and chemotherapy. We have established several tumorigenic CD133+ glioblastoma (GBM) stem cell lines, which are directly derived from patients'primary tumors that are recurrent and had previous treatment. Functional and molecular studies revealed that the slow-growing CD133+ GBM stem cells expressed a series of tumor suppressor/quiescence associated genes but are capable of clonal self-renewal and spontaneous reentry to the cell cycle to generate highly proliferative CD133- progeny that can populate tumor spheres in cultures and reconstitute a malignant tumor in mouse brain. We therefore hypothesize that GBM stem cells use reversibility of cellular quiescence to escape treatment followed by regenerating a new tumor upon treatment removal. We will perform a loss-of-function RNA interference screen for molecular targets of GBM tumor stem cell quiescence, and test whether knockdown of quiescence factors will improve the treatment efficacy of radiotherapy and chemotherapy. Thus, the innovative treatment strategy proposed here aims to redirect reversible, viable arrested tumor stem cells toward non-reversible senescence, apoptosis, or terminal differentiation upon radiotherapy or chemotherapy. Preventing tumor stem cells from reentering cell division cycle after treatment shall greatly diminish the recurrence rate of GBM tumor. Public Health Relevance: Patients with relapsed malignant glioblastoma (GBM) have no effective treatment. Our research goal is to develop a novel therapeutic strategy that can specifically target the quiescent GBM stem cells, which are capable of un-limited self-renewing, generating drug resistance, and giving rise to secondary tumors. Our innovative approach is to destroy the factors that maintain the GBM stem cell quiescence and redirect cells toward a non-reversible senescence, apoptosis, or terminal differentiation after exposure to chemotherapy and radiation treatments, leading to the loss of tumorigenic potential.